Optical fibers are essentially ultra thin light conduits. Light is pumped into one end, propagates forward within and through the fiber, whether bent or straight, and ultimately emerges at the other end. By pumping light into the fiber in a predefined pattern, huge amounts of information can be communicated over large bandwidths over long geographic distances almost instantaneously (i.e., at the speed of light). Thin, fast, and robust, the utility of optical fibers is beyond question.
While the variety, forms, and complexity of fiberoptic configurations continue to evolve, the central underlying structure found in virtually all optical fibers is a light transmitting core surrounded by a cladding layer. The indices of refraction of the core and the cladding are adjusted during manufacture to provide the cladding with an index of refraction that is less than that of the core. When light is pumped into the fiber core, it encounters the refractive index differential at the core/cladding interface and in an optical phenomenon, also referred to as "continuous internal reflection", is "bent" back with little loss into the core, where it continues to propagate down the optical fiber.
In manufacture, an optical fiber is typically drawn from an optical fiber preform that essentially has the same cross-sectional geometrical arrangement of core and cladding components as that of the final optical fiber, but a diameter several orders of magnitude greater than that of the fiber. One end of the preform is heated in a furnace to a soft pliable plastic consistency, then drawn lengthwise into a fiber having the desired fiber core/cladding dimension.
In the art of fiber preform manufacture for transmission fibers, as opposed to the manufacture of active fibers, i.e., fibers with rare earth doped cores in single mode or double clad fibers, techniques have been developed for high speed manufacture to reduce costs while providing high quality fiber using chemical deposition processes where constituents in their vapor phase are supplied to a horizontally rotated refractory tube to form one or more inner glass layers on the inside surfaces of the tube. Examples are the MacChesney at al. patent 4,909,816 and its companion patents, 4,217,027 and 4,334,903, disclosing what is referred to as the modified chemical vapor deposition (MCVD) process, named as such to distinguish it from general semiconductor type CVD processes as well as from prior CVD process employed for the manufacture of glass preforms. These patents discusses the so called "soot" or outside vapor deposition (OVD) process, disclosed in U.S. Pat. Nos. 3,775,075 and 3,826,560, which process is enhanced by the use of the MCVD process. Patent '816 relates to the establishment of a more prominate homogeneous reaction where the reaction product from the vapor phase forms glass precursor particulates within the gas stream within the ambient of the refractory tube which particulates are then subsequently deposited downstream of the heat zone or source on the inner surface of the tube. The deposited particulates are then consolidated into a transparent glass layer on the tube surface by the passing heat zone. This is distinguished from previously employed CVD processes for glass preforms where a heterogeneous process is explained to occur with glass particulates initially formed on the inner surface of the refractory tube forming either a soot layer that is subsequently sintered to form a glass layer or directly forming a glass layer resulting in deposited formation of monolithic glass, as opposed to glass particulates initially formed within the ambient of the glass refractory tube. The homogeneous reaction of the MCVD process is accomplished, in a significant manner, by increasing the temperature of the reaction zone via the hot moving zone. The advantage of the MCVD approach over the OVD process is eliminating hydrogen bearing components, water vapor and other contaminants from the deposited glass layer. The MCVD process is explained briefly in Andrejco et al. U.S. Pat. No. 4,257,797 and is explained in detail in the book entitled "Optical Fiber Communications", Vol. 1, Fiber Fabrication, edited by Tingye Li, 1985 (Academic Press, Inc.), in particular, at pages 1-64, which is incorporated herein by reference.
From the point of view of patent '816, higher productivity of glass preforms for large scale fiber production, via subsequent fiber drawing, can be achieved by providing a continuous, unbroken processing procedure which includes increasing the reaction temperature for glass layer formation, increasing the rate of tube rotation, sintering the deposited glass layer, minimizing the effects of hydration contamination from the deposited glass layer while rotating and collapsing the tube to form the preform. While the high speed process approach may be highly applicable to manufacture transmission fiber, it is not a preferred approach to the manufacture of active fiber, particularly where high levels of a rare earth dopant or codopants are desired for incorporation in the deposited layer or layers on the inner surface of the refractory tube. Active optical fibers are employed as fiber gain media for purpose of signal amplification of fiber laser applications and are comprised of a single mode fiber or a double clad fiber with a core composition doped with 4f rare earth elements (i.e., the lanthanide series of element, atomic numbers 57-71), e.g. erbium or ytterbium or co-doped with erbium and ytterbium. By selective use of particular concentrations and/or mixes of rare earth dopants, the spectral absorptivity of the core to certain wavelength ranges of light can be defined to desired specifications. An appropriately tuned core, surrounded with an appropriate cladding configuration, can provide, in combination with an appropriate pump source, the basis for light lasing and/or light amplifying functionality. In consideration, for example, of the need for signal amplification in fiberoptic telecommunication projects, optical fibers capable of such light intensifying functionality are desirable. Unfortunately, rare earth doping is not easy performed, particularly at high levels of concentrations in the core.
Various methods and variation have been developed for fabricating rare earth doped optical fiber preforms. Some examples of these methods are disclosed in U.S. Pat. No. 4,501,602, issued to Miller et al. on Feb. 26, 1985; U.S. Pat. No. 4,616,901, issued to MacChesney et al. on Oct. 14, 1986; U.S. Pat. No. 5,236,481, issued to Berkey on Aug. 17, 1993; U.S. Pat. No. 5,609,665, issued to Bruce et al. on Mar. 11, 1997; U.S. Pat. No. 4,501,602, issued to Miller et al. on Feb. 26, 1985; U.S. Pat. No. 4,826,288, issued to Mansfield et al. on May 2, 1989. Regardless, under current practice, it is very difficult to incorporate high concentrations of rare earth dopants at limited total doping levels, particularly, in the case of the popular rare earth element neodymium (Nd).
Part of the problem is the need in one of the most commonly-practiced preform manufacturing methodologies, i.e., MCVD, to generate and deposit as layer a vapor laden with rare earth dopant. Under current practice, it is difficult to generate anything other than relatively low vapor pressures, resulting ultimately in the incorporation of correspondingly low concentrations of rare earth dopant. Without the ability to attain a high rare earth dopant concentration, one cannot produce an optical fiber with a low numerical aperture, a low core attenuation, and high pumping power absorption, all of which are desired criteria in the design of fiberoptic lasers and amplifiers.
Additionally, in regard specifically to fiberoptic lasers, even if a suitable fiber optic preform is made, the lasing efficiency of a fiber drawn therefrom may still suffer in other respects. The performance of fiber lasers, as in any active or nonlinear waveguide, is related intimately to the efficiency with which pump radiation can be absorbed by the active material in the fiber core. In the earliest fiber lasers, an appreciable amount of the radiant energy pumped into the fiber would not pass into the core, and, thus, did not contribute to the core's lasing effect. In response, various cross-sectional fiberoptic geometries, in particular, pertaining to the cross-sectional geometry of the inner cladding of a double clad fiber, were successfully developed that are capable of effecting patterns of internal reflection having a greater frequency of core interactions with light propagating along the inner cladding and criss-crossing and being absorbed in the doped core. See, for example, U.S. Pat. No. 4,815,079, to Snitzer et al. issued Mar. 21, 1989; and U.S. Pat. No. 5,533,163, to M. H. Muendel issued Jul. 2, 1996. However, designing a fiber on the basis of such learning requires additional manufacturing steps in the preform formation. Any improvement that would reduce the burden of these additional steps with the enhancement of light scattering in the inner cladding for enhancing absorption in the fiber core would be desirable.